[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > This will introduce breaking changes. I think it is fine enough to emit warning here.
> 
> Do you feel strongly about that? FWIW, GCC and MSVC have both implemented it as an error.

Yeah, we should avoid breaking changes if possible. It is bad user experience. Especially the forbidden code is not bad actually.

https://github.com/llvm/llvm-project/pull/194201